The present invention relates to a system equipped with a microcomputer including an AD (analog-to-digital) converter and mounted on a power tool body, a battery pack for electric power tool or a charger for electric power tool. The present invention also relates to a battery pack for electric power tool equipped with the above system.
In recent years, a battery pack for electric power tool which includes various types of rechargeable batteries, such as a nickel hydride rechargeable battery, a lithium ion rechargeable battery and others, are equipped with a microcomputer to achieve high functionality and high efficiency. The microcomputer is known to control charging/discharging of the rechargeable battery based on various information such as the voltage, temperature and others of the rechargeable battery, and detect overcharging/overdischarging of the rechargeable battery.
Not just a battery pack but a widely used charger for electric power tool which supplies charging power to a battery pack for charging a rechargeable battery, or power tool body which receives power supply from a battery pack to operate, is also equipped with a microcomputer and configured to perform various control by the microcomputer.
The microcomputer installed in a battery pack, a charger for electric power tool and a power tool body generally includes an AD converter. In the battery pack, for example, various information (analog signals) such as the voltage, temperature, and so on of a rechargeable battery inside the battery pack are inputted from an AD port of the microcomputer to the internal AD converter and converted to digital data by the AD converter. The microcomputer determines various conditions inside the battery pack including the rechargeable battery, and based on the conditions, performs various controls such as charging/discharging control and detection of overcharging/overdischarging.
In the battery pack and the charger in which various controls are performed by the microcomputer including the AD converter, if an AD converter inside the microcomputer itself fails or any abnormality occurs to the AD port of the microcomputer due to adhesion of dust, and the inputted analog signal is no longer converted to digital data correctly (hereinafter, collectively referred to as “AD abnormality”), various controls by the microcomputer may not be performed correctly.
For example, it is assumed that the AD abnormality occurs when the microcomputer receives the voltage of the rechargeable battery via the AD converter and controls charging based on the voltage value. Then, the microcomputer becomes unable to correctly recognize the voltage of the rechargeable battery, e.g., the AD converter may incorrectly convert the voltage of the rechargeable battery to 2V although the rechargeable battery is actually charged to 3V. In such case, the microcomputer may continue charging although the rechargeable battery is already fully charged. The rechargeable battery may be overcharged.
In order to protect the rechargeable battery from malfunctioning of the microcomputer deriving from the AD abnormality as such, a protection IC (which is called a second protection IC) is provided separately from the microcomputer, which detects the voltage of the rechargeable battery to detect overcharging/overdischarging in a conventional method (see APPLICATION CIRCUIT diagram in “Protection for Lithium-Ion Batteries (2-serial cells) Monolithic IC MM3112 Series” by MITSUMI ELECTRIC CO., LTD., <http:www.mitsumi.co.jp/Catalog/pdf/battery_mm—3112_e.pdf>)
The protection IC detects the voltage of each battery cells of an assembled battery in which a plurality of battery cells (rechargeable batteries) are connected in series. In case that overcharging or overdischarging is detected in any one of the battery cells, protection operation is performed to forcibly stop charging/discharging. The protection operation is performed by interrupting a current path by turning off a FET connected between a positive terminal of the battery pack and a positive terminal of the rechargeable battery (positive terminal of the battery cell having a highest potential).
Accordingly, even if the voltage of the rechargeable battery is not correctly AD converted due to the AD abnormality in the microcomputer, the rechargeable battery can be protected by such protection IC when the rechargeable battery is brought into an overcharged/overdischarged state.
In the above-described protection method utilizing a protection IC, the AD abnormality cannot be detected unless the rechargeable battery is in (or close to) an overcharged/overdischarged state. Therefore, upon occurrence of the AD abnormality, the rechargeable battery has to be out of the normal voltage range at least temporarily.
Specifically, the protection IC does not detect the AD abnormality in the microcomputer, but checks the actual voltage of the rechargeable battery. If the voltage is higher than normal (i.e., the rechargeable battery is in or close to an overcharged state), the protection IC determines that the rechargeable battery is in an overcharged state and activates the protection operation. Accordingly, even if the AD abnormality occurs to the microcomputer and the voltage of the rechargeable battery is not correctly AD converted, the protection operation is not activated immediately. After the charging voltage of the rechargeable battery is gradually increased to an overcharged state due to the AD abnormality, the protection IC activates the protection operation. Thus, it is inevitable that the voltage of the rechargeable battery at least temporarily goes out of the normal range.
Also, since the protection IC is separately provided from the microcomputer, a space for installing the protection IC becomes necessary. The battery pack becomes large in size. There is an increase in costs as a whole including the cost for the protection IC itself.
Not only in the battery pack but also in the microcomputer mounted on a charger or a power tool body, normal control by the microcomputer may not be performed if the AD abnormality occurs.